Plasticity of Clay in Acidic Environment

[luke648]

I am currently working on a project in a chemical plant evaluating what appears to be heaving soils. Concrete paving is heavily cracked, tanks in the area are leaning, etc. In this particular area, there had been a problem with acidic water (sulfuric acid used in process)leaking onto the area. A couple of tanks in the area have been in place since 1966, some are a little newer. Some movement was noticed around 2000, and the worst seems to have occured between that time and 2004. A survey of foundation points was initiated in 2004; however, a welding company was hired to do the survey, and there was little control (i.e. survey points, personnel, condition of benchmark, etc.) and the numbers are, to us, inconclusive. At some point between 2000 and 2004, the acidic runoff was diverted to drainage ditches. Aggravating the issue, there whave been some significant personnel changes, and some of the people in place now can't answer some of the questions about when and if operational changes were made that changed the environment, etc.

The tanks are founded on shallow footings bearing about 4 feet below finished grade. We have have one boring in the immediate area and one 30 feet away out of the process area - and some lab analyses...foundation zone soils near immediate area have high moisture content (24 to 31), sandy clay and clayey sand, LL from 24 to 34, PI from 10 to 22. In a couple of cases the moisture is approaching the LL, but not all. Perched water encountered 3 feet below finished grade. Soil pH runs between 3.3 and 3.5, while water tested at 2.7. Soil pH in the removed area runs between 5 and 8.

One thing we were told is that they had a similar problem in a different area that had been subject to acidic water. In doing some research, I have found some references that indicate clays exposed to acidic soil conditions do tend to show increased swelling in plastic soils. The chemistry is way beyond me, though!

I guess that my question is, has anyone had any experience with clays in acidic environments? If you have, how was it handled? Is this train of thought way off base? There is some argument here that we could be looking at a settlement issue, not heaving although in the field it looks like heaving. Timeframe is now limited due to internal plant schedules, so we don't have time to set up a long-term monitoring program and are looking at undercutting all the clay in the area and backfilling with flowable fill or lean concrete - it will have to be a special mix for acid resistance.

Sorry to be so lengthy, but I felt that all this information was important! Thanks in advance.

 

[BigH]

Luky648 - please don't double post. I would presume almost all geotechnical engineers monitor all geotechnical and structural threads.

 

[luke648]

BigH - my apologies. I am afraid I am not familiar enough with the forum, and since it was an oddball topic, I was hoping for as much exposure as possible - especially civil/environmental folks who may not monitor geotech/structural posts! Anyway, I am still looking forward to any experiences anyone is willing to share. It is my hope that as I am gaining more experience I can do the same for others.

 

[dik]

Luke648

Don't feel too bad; I've sometimes posted in a couple of forums (fora?). Regarding your question, I've asked some of my aquaintances about the change in PI with acid... still no answer...

With your querie, I do know that highly plastic clays have significant charges on the 'platlets' that attract/retain polar water molecules and that an acid liquid could modify the electrolytes in the soil, making it more conductive and thereby increasing the adsorbed water. At a molecular level, plasticity is largely an electrical issue. This may have an effect on the heave susceptibility if it allows water to form lenses by increasing the permeability (bigger voids provided by increased water molecules or because there's more water. But it's a bit of a guess.

As an aside, with the platlets closely packed, ie. with just a single bonded layer of water it is possible that because of the impervious nature of plastic clays that water can remain in liquid state at many degrees below zero (ie, -40) because it can't undergo the volume change required to change state. Clay by itself may not necessarily be frost heave susceptible.

Dik

 

[luke648]

Dik, thanks for the comments. The study I found focused on the effect of acidic liquids discharged during mining processes on clay liners of disposal sites. The study agrees with what you stated in that water adsorption was increased due to changes in the bonds due to the effects of the acidic solution. It also indicated that the swelling index was increased at higher concentrations of acid, although possibly only temporarily. It has been a long time since Chemistry 101 for me!

We were unable to perform a swell test of the affected clays, but the plasticity indices did not indicate a highly plastic soil although I realize that low PI does not necessarily preclude a high swell index. That's an interesting point about heave...I should mention that the project is located in the deep south; however, the occasional cold snap can bring temperatures in the mid-to- upper 20's for short periods of time (2 days)altough not much lower than that.

 

[dik]

Highly plastic clays are subject to swelling as well as shrinkage. If additional water can be adsorbed, then the swelling characteristics are likely enhanced. Interesting problem, though...

Dik

 

[luke648]

Dik, thanks. It sure is interesting. I guess the moral to the story is to practice good housekeeping any keep any kind of runoff away from foundations! I will post any developments. Thanks again.